Results from offshore controlled-source electromagnetic (“CSEM”) surveys, such as those collected using the methods disclosed in U.S. Pat. No. 6,603,313 to Srnka and U.S. Patent Publication No. 2003/0050759 published Mar. 13, 2003 (Srnka, et al.), have shown that the resistivity in the earth can be strongly dependent upon the direction of the electrical current flow used to make these measurements. In particular, the vertical electrical resistivity can be much (two or more times) larger than the horizontal electrical resistivity, especially in finely layered rocks such as shales, and can vary in magnitude from location to location. This phenomenon is called electrical anisotropy, or specifically electrical vertical transverse isotropy (“EVTI”) by practitioners in the art. The earth's electrical resistivity may also vary azimuthally (i.e. in the compass directions), but this anisotropic effect appears to be generally much less important (i.e. of much smaller magnitude) than EVTI in sedimentary basins of interest for hydrocarbon exploration.
The presence of EVTI distorts the signals received at seafloor electromagnetic receivers used in marine CSEM surveys acquired with a horizontal electric dipole (“HED”) or a horizontal magnetic dipole (“HMD”) controlled source, relative to that which would be received in the absence of EVTI. This distortion affects the interpretation of marine CSEM resistivity anomalies associated with the presence of reservoired hydrocarbons. Such distortion effects appear in both the amplitude and phases of measured seafloor fields, and change with frequency. These distortions can mask the presence of hydrocarbons (false negatives) or incorrectly suggests their presence (false positives). Distortions of this kind have been observed in numerous CSEM surveys.
Marine CSEM surveys for hydrocarbon applications have been acquired using HED controlled sources because of their operational advantages and superior coupling of energy into the earth. (Chave, et al., “Electrical Exploration Methods for the Seafloor,” Electromagnetic Methods in Applied Geophysics 2, 931-966, Soc. Expl. Geophysics, Tulsa (1991)). HED sources produce both vertical and horizontal electrical currents in the earth. HMD sources also produce both vertical and horizontal electrical currents, but to date have not been used for offshore hydrocarbon applications because of their low power and other operational constraints. The vertical electric dipole (“VED”) method (Edwards, et al., J. Geophys. Res. 86B, 11609-11615 (1981)) produces primarily vertical currents in the earth, but with much lower efficiency (poorer coupling) than HED sources. The vertical magnetic dipole (“VMD”) source produces essentially only horizontal earth currents, and to date has also not been used in marine CSEM surveys due to operational disadvantages. The measurement of both online and offline (“broadside”) horizontal inline (Ex) and crossline (Ey) seafloor electric field components that measure earth responses from an HED source is known in the art of marine CSEM surveys for structural studies.
It is well known to practitioners of the art that the earth's electrical resistivity can be anisotropic. See, for example, Keller and Frischnecht, Electrical Methods in Geophysical Prospecting, 33-39, Pergamon (1966); Kaufmann and Keller, Frequency and Transient Soundings, 257-284, Elsevier, N.Y.(1983); Negi, et al., Anisotropy in Geoelectromagnetism, Elsevier, N.Y. (1989); Zhdanov and Keller, The Geoelectrical Methods in Geophysical Exploration, 119-124, Elsevier, N.Y. (1994). Several authors teach how to calculate (model) the anisotropic earth electrical responses for various controlled sources. See, for example, Chlamtac and Abramovici, Geophysics 46, 904-915 (1981); Yin and Weidelt, Geophysics 64, 426-434 (1999); Yin and Maurer, Geophysics 66, 1405-1416 (2001). Also, several authors discuss the interpretation of azimuthal electrical anisotropy (for example, Watson and Barker, Geophysics 64, 739-745 (1999); and Linde and Peterson, Geophysics 69, 909-916 (2004)). Others discuss the interpretation of EVTI (Jupp and Vozoff, Geophys. Prospecting 25, 460-470 (1977); Edwards, et al., Geophysics 49, 566-576 (1984); and Christensen, Geophys. Prospecting 48, 1-9 (2000)) using a variety of controlled sources. Tompkins et al., (“Effect of Vertical Anisotropy on Marine Active Source Electromagnetic Data and Inversions,” EAGE 65th Annual Convention, Paris, France, abstract E025 (2004)) describe several effects of EVTI in marine CSEM data collected for hydrocarbon applications, using only (seafloor) electric field measurements.
Jupp and Vozoff (citation above) describe the use of onshore CSEM and magneto telluric (MT) data to estimate EVTI. They used zero-frequency (DC) controlled-source HED data measured only on the source line, and did not discuss the case of offshore applications at or near the seafloor where the electromagnetic responses are quite different from onshore. DC controlled-source resistivity data are inline static electric field values measured at various distances from the grounded HED source along the source line, and are sensitive to both vertical and horizontal resistivities as discussed in other references cited herein. Jupp and Vozoff show, using synthetic data, that the EVTI can be determined from data that are sensitive to only the horizontal resistivity combined with the DC HED data. MT data have this sensitivity only to the horizontal resistivity, which has been well known in the art. Jupp and Vozoff describe a one-dimensional inversion algorithm that uses the DC HED and MT data to successfully solve for the EVTI.
The published efforts to quantitatively determine the extent of the EVTI effect (such as Chlamtec) attempt to do so using conventional CSEM data which is sea-bottom measurements of electric field components, usually horizontal components. None of them propose particular data acquisition techniques such as the use of certain source and receiver combinations and the measurement of other electromagnetic field components such as Hz in conjunction with subsequent data processing steps to assess EVTI. None of the above-mentioned publications disclose the use of vertical magnetic field (Hz) measurements in combination with electric field measurements, in order to determine the EVTI. However, the use of Hz data for electromagnetic prospecting on land is well known, for example using the Tipper value in magneto tellurics to detect 3D structure (Kaufman and Keller, The Magnetotelluric Method, 483-484, Elsevier (1981)), or using Hz data collected at the center of a vertical magnetic dipole (VMD) source in the central loop induction method for resistivity depth sounding (Zhdanov and Keller, The Geoeletrical Methods in Geophysical Exploration, 396-411, Elsevier (1994)).
Instead, the published literature tends to suggest that the useful onshore response of Hz may be replaced by Ez responses offshore in the presence of deep seawater (Berdichevsky, et al., Marine Deep Geoelectrics (in Russian), Nauka, Moscow (1989); Golubev and Zhdanov, Consortium for Electromagnetic Modeling and Inversion Annual Report, 175-217, U. Utah (1998)). Although Cheesman et al. (Geophysics 52, 204-217 (1987)) show calculated offline Hz values for an HED seafloor source, they do not disclose its use in combination with online seafloor Ex, Ey, or Ez signals, nor do they discuss use of Hz for determination of EVTI.